The invention relates to the design of structures with near zero thermal expansion or other design-specific thermal shape change characteristics using materials which, although desirable for other reasons, would not be acceptable with conventional single-material structural design because their intrinsic thermal expansivity doesn't match the design criteria.
In the design of most engineering structures which are subject to significant temperature excursions, the thermal expansion behavior of the structure is of key importance. The constituent material's coefficient of thermal expansion (CTE) is one of the driving material properties considered in the engineering materials selection process. The importance of the thermal expansion stems from several basic types of design requirements. In applications, such as supports for space-based mirrors, the dimensional stability is a key consideration. That is, ideally the structure should exhibit very little dimensional change when subjected to substantial changes in temperature that occur as the structure is exposed to changing environmental radiation conditions. Prior attempts to design stable structures for this type application resulted in only one-dimensional thermal stability and required complex pin-jointed designs. Another class of design challenge arises in structures subjected to space-varying thermal gradients, such as engine components subjected to hot combustion environment or hypersonic airframe surfaces subjected to aerothermal heating. The resulting gradient in thermal strain can result in design-limiting thermally induced stressing. In such cases, stresses may be reduced by specifically varying the material's CTE for compatibility with the thermal gradient.
The ‘menu’ of intrinsic CTE's available with structural engineering materials is however quite limited, and in most cases, available materials with the ideal expansivity for an application are less than ideal for other reasons. For example, intrinsically low expansion glasses currently used for space mirror supports are inferior to ceramic composites in terms of weight, fabricability and ultimate temperature capability. As another example, ultra high temperature ceramics (UHTC's) appear to be ideally suited to hypersonic leading edge applications, however their expansivity is too large for compatibility with the much cooler support structure. It is therefore desirable to design composite structures with effective expansivities that are substantially different from that of the constituent materials.
The possibility of controlling the thermal expansion will allow great design flexibility, and is the basis of this work. The manner in which thermal expansion influences design varies with the specific application. A few current applications of high technological interest are outlined below, before presenting an exploration of possible solutions for tailored structures.
An example of one approach is shown in Schuerch, H. U. Thermally stable macro-composite structures, NASA Contractor report CR-1973, February 1972 and incorporated herein by reference. Another approach is discussed in Steeves, C. A., dos Santos e Lucato, S. L. He, M. Antinucci, E., Hutchinson, J. W. and Evans, A. G. Concepts for structurally robust materials that combine low thermal expansion with high stiffness Journal of the Mechanics and Physics of Solids 55, 1803-1822 (2007) also incorporated herein by reference. Both have devised an isotropic pin-joint approach.
Another class of design challenge arises in structures subjected to space-varying thermal gradients, such as engine components subjected to hot combustion environment or hypersonic airframe surfaces subjected to aerothermal heating. The resulting gradient in thermal strain can result in design-limiting thermally induced stressing. In such cases, stresses may be reduced by specifically varying the material's CTE for compatibility with the thermal gradient. Additionally, structures which are specifically designed to undergo significant thermally-driven shape change may be utilized as actuators or actively deforming aero structures.
With these motivations, a composite material/structural design concept was explored to determine the merits and limitations of such concepts. In particular, the possibility of devising structure that allows fabrication of structures with tailored or designed-in CTE values is examined. Analytical expressions are derived that will enable designing with these structures practical. The derivations may be verified using finite element methods.